1. Field of the Invention
This invention relates to a waveform processing system for reproducing magnetically recorded digital information, and its waveform processing method, and particularly to a system and a method of this type which can detect the bits of the digital information on the basis of a reproduction waveform.
2. Description of the Related Art
In recent years, with the spread of personal word processors and personal computers, many floppy disks having a size of e.g. 3.5 inches have arrived on the market as an external memory device for storing digital information.
A longitudinal recording method is applied to such a floppy disk. This disk, which is indicated by reference numeral 10 in FIG. 1, has a magnetic layer 11 provided with longitudinal magnetized blocks 12a and 12b. In this case, a magnetic charge is concentrated in a boundary area between the blocks 12a and 12b since the directions of the magnetization acting in these blocks are opposite to each other. Thus, if reproduction of the recorded information is performed using a ring head 13, a reproduction waveform el having a maximum output (b) at the boundary between the blocks 12a and 12b is obtained. Similarly, the waveform el has other maximum outputs (a) and (c) at the boundaries between blocks 12a and 12c and between blocks 12b and 12d, respectively (the directions of the outputs, however, are opposite to that of the output obtained between the blocks 12a and 12b). Points at which the direction of magnetization is changed are detected by detecting those peaks (a), (b), and (c) of the reproduction waveform el at which the differentiated values are 0 (the points of these peaks are hereinafter called "differentiated value zero-cross points"), thereby detecting the bits of the reproduced information (one peak point=one bit "1").
In the above-described longitudinal recording however, the magnetization is minimum at the magnetic direction-changing point since demagnetizing field is also maximum at the point. This being so, in the case of high-density recording, demagnetizing field will be conspicuous, so that a sufficient reproduction output will not be obtained.
To avoid this, a perpendicular magnetic recording method has been proposed and put into practice in place of the longitudinal method. In the perpendicular magnetic recording, recorded information is reproduced from magnetization acting in the thickness direction of the magnetic layer, as is shown in FIG. 2. In this method, adjacent magnetized blocks 22a-22d of a floppy disk 20 cooperate to strengthen their magnetic forces, which is optimal to reproduce high-density records.
Digital information is recorded in the magnetic layer 21 of the recording medium 20 in its thickness direction, as is shown in FIG. 2. When the information is reproduced by a ring head 23, zero-cross points (a), (b), (c), (d), and (e) of a reproduction waveform e2 are magnetization direction-changing points, so that the bits of the digital information recorded are detected by detecting the zero-cross points (one point =one bit "1").
However, in the perpendicular recording method, since the magnetization formed in the magnetic layer 21 is not completely perpendicular but contains a perpendicular vertical component V and a longitudinal component L. Specifically, as is shown in FIG. 3, the actual waveform reproduced from the magnetic layer 21 differs from the ideal perpendicular waveform e2 shown in FIG. 2, but is identical to that e2' superposed with a longitudinal waveform component upon a perpendicular waveform component.
Thus, the zero-cross points (a'), (b'), (c'), (d'), and (e') of the reproduction waveform e2' deviated from the actual magnetization direction-changing points (a), (b), (c), (d), and (e). If the reproduced zero-cross points (a'), (b'), (c'), (d'), and (e') much deviate from the actual magnetization direction-changing points (a), (b), (c), (d), and (e), the difference in the phase of the reproduction waveform e2' and that of reproduction clock signal will be large, whereby
reproduction clock signal will be large, whereby accurate bit detection will not be performed.
If the longitudinal component contained in the reproduction waveform e2' is represented by fH(t), and the perpendicular waveform component fP(t), the following equation (1) is given: EQU fH(t)=-fP(t)[1/(t-.tau.)]d.tau.... (1)
A method was contrived in which the perpendicular component fP(t) was removed from the waveform e2' by using the equation (1), i.e, only the longitudinal component fH(t) was extracted, thereby obtaining a waveform having zero-cross points close to the actual magnetization direction-changing points (a)-(e).
To execute this method, for example, a Hilbert filter 101 shown in FIG. 4 has been used in a waveform processing circuit. The filter 101 comprises an input terminal 103, a delay line 107, a plurality of resistors Ri (i=1, 2, ..., N) connected to the line 107, an operational amplifier 106, and an output terminal 105 connected to the amplifier 106. In the circuit incorporating the Hilbert filter, however, lots of delay time is required for performing accurate Hilbert conversion, and hence a long delay line is required Therefore, providing the Hilbert filter 101 constructed as above in a driving device for driving a floppy disk is not desirable since it makes the apparatus large.